The present invention relates to apparatus for and a method of bending strip metal material such as I section beams and U section beams or other formed sections.
In order to bend such sections it is conventional to provide three or more rolls which straddle the feed path of the material to be bent. The first roll normally has a fixed axis on one side of the path, the next or second along the path is on the other side, whilst the third roll is on the same side of the path as the first, the second and third rolls have axes which are adjustable with respect to the bending path. Other rolls may also be provided.
The axes of the rollers are essentially parallel (or sometimes on an angle in order to generate a cone shape) to each other, and are arranged in approximately a triangular pattern so that a strip or sheet like workpiece passed between them is bent in to a curved shape. Such a bending machine will hereafter be referred to as a bending machine of the type described.
A conventional roll bending machine is driven by a single motor and/or gearbox. One or more of the rollers may be driven directly or by shafts, gears, chains etc. and consequently all driven rollers rotate at the same speed or fixed ratio of speeds. One or more rollers may not be driven at all and allowed to freewheel.
When bending thin sections or large radius curves this does not present a problem but with small radii and thicker sections the different relative speeds of the outside and inside of the curve results in either slip, distortion of the section or overloading of the gearbox components. When bending large sections this problem is kept to a minimum by passing the workpiece backwards and forwards several times making progressive reductions in radii and permitting some slip on one or more of the rollers.
If a small radii is attempted in a single pass of the workpiece, the internal forces induced in the workpiece will cause severe and unacceptable deformation. On the other hand, if one or more of the rollers is allowed to freewheel to eliminate slip, then the driven roller(s) will also slip and/or distort the section.